A standard biax plug-in lamp, e.g. a fluorescent lamp intended to fit in a socket of the G23 or G24 type, has a base from which a normally square plug part projects axially, with several contact pins projecting from the base parallel to the axis next to the plug part. The socket for such a lamp has a socket hole complementary to the plug part and, to the sides of the hole, respective contact holes into which the contact pins fit when the plug part is engaged in the socket hole, with metal contacts in the contact holes making the necessary electrical connection to the contact pins. The socket hole is dimension to fit only with the plug part of a lamp with the right rating, and often complementary index ridges and grooves are provided in the socket hole and on the plug part to ensure that only a lamp exactly right for the fixture can be fitted to the socket.
In U.S. Pat. No. 6,340,310 of Henrici such a socket is disclosed having spring-loaded gripping fingers that engage behind retaining bumps on the plug part when the plug part is fully inserted in the socket hole. This interfit ensures that the lamp will be gripped solidly enough that it will not fall out of the socket if installed on a ceiling, and that in general accidental light engagement or vibration will not cause the lamp to work free.
In U.S. Pat. No. 5,746,615 the socket is constructed such that the lamp can be inserted into the socket and then twisted to bring retaining bumps on the plug part under complementary retaining formations on the socket part, that when twisted into the right position lock the lamp in place. This structure is fairly complex in that it is necessary to provide special contact systems to grip the contact pins when they are pivoted into the locked position. In addition the socket is somewhat bulky since the lamp must be able to pivot in it, and even so the lamp can work loose if it is inadvertently given a pull or subjected to substantial vibration.